The lipid peroxidation product 4-hydroxynonenal (4-HNE) is a signaling molecule affecting several fundamental biological processes, including aging. In generating a knockout mouse with impaired 4-HNE metabolism, we found that elevated 4-HNE causes an obese phenotype. We also observed increased fat accumulation caused by either silencing of 4-HNE metabolism or by direct exposure to 4-HNE in the nematode C. elegans, establishing that 4-HNE-triggered fat accretion is phylogenetically conserved. In mice, elevated 4-HNE is associated with age-dependent emergence of insulin resistance, a condition that could curtail life span. We directly observed shortened life span in C. elegans with 4-HNE-triggered fat accumulation. Therefore, interference with 4-HNE signaling that would otherwise cause fat accumulation has the potential not only to alleviate obesity, but also to prevent conditions with high public health impact such as insulin resistance and diabetes, as well as age-associated diseases. Before interventions can be devised to counteract these detrimental effects of 4-HNE, it is necessary to understand the mechanisms by which 4-HNE contributes to obesity. We hypothesize that these mechanisms include activation of acetyl-CoA carboxylase (ACC), and have already confirmed the resulting prediction of elevated malonyl-CoA in mice and worms with increased tissue levels of 4-HNE. It is known that elevated malonyl-CoA leads to obesity via an increase in fatty acid biosynthesis and inhibition of fatty acid beta-oxidation. We will test our hypothesis that 4-HNE modulates ACC activity, in two complementary experimental systems: knockout mice (because of similarities between murine and human physiology) and C. elegans (because of its genetic and biochemical tractability, facile aging studies, and general conservation of fat-related metabolism between nematodes and mammals). We will then test the major predictions following from this hypothesis in human cell lines. The Specific Aims of the proposed studies are (1) to measure, in mouse tissues, changes in metabolite levels and enzyme activities to determine whether 4-HNE modulates ACC by increasing its allosteric activators such as citrate, and/or by decreasing ACC phosphorylation. The life span of mice with increased 4-HNE levels will be also determined; (2) to manipulate genetically C. elegans enzymes that modulate malonyl-CoA to define the precise mechanism of 4-HNE action and to determine whether the effect of 4-HNE on longevity is mediated by, or independent of fat accumulation; and (3) to test whether the mechanism of 4-HNE-triggered fat storage observed in animal models applies to human biology